Transcript Metal Ion Indicators

EDTA Titration
EDTA = Ethylenediaminetetraacetic acid
Lewis acid : electron pair acceptor eg metal
ions
Lewis base : electron pair donor eg ligands
A ligand that
attaches to a metal
ion through more
than one ligand
atom is called a
chelating ligand or
multidentate
ligand
A titration based on complex formation is called
complexometic titration
The equilibrium constant for the reaction of a
metal with a ligand is called the formation
constant, Kf , or the stability constant
Consider :
Ag+ + NH3
Ag(NH3)+ + NH3
Kf1 =
[Ag(NH3)+]
[Ag+][NH3]
[Ag(NH3)2+]
Kf2 =
[Ag(NH3)+][NH3]
Ag(NH3)+
Ag(NH3)2+
The overall reaction :
Ag+ + 2NH3 = Ag(NH3)2+
and the overall formation constant :
Kf =Kf1 Kf2 =
[Ag(NH3)2+]
[Ag+][NH3]2
Example :
A divalent metal M2+ reacts with a ligand L to
form a 1:1 complex :
M2+ + L
ML2+
Calculate the concentration of M2+ in a solution
prepared by mixing equal volumes of 0.20 M M2+
and 0.20 M L. Given Kf = 1.0 x 108
Given Kf = 1.0 x 108
complex is sufficiently
strong such that the reaction is virtually
complete
Since equal volumes were added
concentration is halved
initial
Let x = residual concentration of M2+
M2+ + L
x
Kf =
ML2+
x
0.1 - x
x2
0.1 – x
= 1.0 x 108
x = 3.2 x 10-5 M
Similarly, if L is a multidentate ligand and
M + nL
MLn
then
Kf =
[MLn]
[M][L]n
EDTA Complexes
EDTA is a hexaprotic system – H6Y2+
Neutral acid is tetraprotic – H4Y
It can be represented as having four Ka values :
Ka1 = 1.0 x 10-2
H4Y
H+ + H3Y-
H3Y-
H+ + H2Y2-
H2Y2-
H+ + HY3- Ka3 = 6.9 x 10-7
HY2-
H+ + Y4-
Ka2 = 2.2 x 10-3
Ka4 = 5.5 x 10-11
Hence in complexing with +1 cation :
Ag+ +
Y4-
= AgY3-
Kf =
[AgY3-]
[Ag+][Y4-]
+2 cation :
Hg2+ +
Y4- = HgY2-
Kf =
[HgY2-]
[Hg2+][Y4-]
+3 cation :
Fe3+ +
Y4- = FeY-
Kf =
[FeY-]
[Fe3+][Y4-]
+n cation :
Mn+
+
Y4-
=
MYn-4
Kf =
[MYn-4]
[Mn+] [Y4-]
Fraction of the total EDTA species that exists
as Y4- = a4 = [Y4-]/CH Y
4
where
CH Y = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] +
4
[H4Y]
Substituting [Y4- ] = a4CH
[MYn-4]
[Mn+] a4CH
Kf =
a4Kf =
Y4
Y
4
[MYn-4]
= Kf’
n+
[M ]CH Y
4
Conditional formation constant
– holds only for a particular pH
– describes the formation of MYn-4 at any
particular pH
– allows us to look at EDTA complex formation as
if the uncomplexed EDTA were all in one form
Example :
Consider the reaction: Fe3+ + EDTA
FeY-18 and a
(Kf = 1.3 x 1025; aY (at
4- pH1) = 1.9 x 10
Y
-9
(at pH4)
4- = 3.8 x 10 ). Calculate the
concentration of free Fe3+ in a solution of 0.10 M
FeY- at pH4 and at pH1.
Using Kf’ = a4Kf ,
Kf’ = (3.8 x 10-9)(1.3 x 1025)
at pH = 4 :
=
4.9 x 1016
Kf’ = (1.9 x 10-18)(1.3 x 1025)
at pH = 1 :
=
2.5 x 107
Let x = [Fe3+] = [EDTA], thus
[FeY-]
[Fe3+][EDTA]
x
=
0.1 - x
x2
= 4.9 x 1016 (at
pH4)
= 1.4 x 10-9 M
Similarly, at pH1:
x = 6.4 x 10-5 M
pH affects the stability of the complex.
The Kf’ values show that the metal-EDTA complex
becomes less stable at lower pH
Titration of Ca2+ with EDTA as a function of pH :
For a titration reaction to be effective, the
equilibrium constant must be large
the
analyte and titrant should be completely
reacted at the equivalence point
EDTA Titration Curves
Titration is carried out by adding the chelating
agent to the sample :
Mn+ + EDTA MYn-4
Kf’ = a4Kf
The titration curve is a graph of pM (= -log[M])
versus the volume of added EDTA
Titration curve consists
of 3 regionsBefore the equivalence
point :
There is excess Mn+ in
the solution after the
EDTA has been
consumed.
Concentration of free
metal ion =
concentration of excess
unreacted Mn+
At the equivalence point :
[Mn+] = [EDTA]
Free Mn+ is from the dissociation of MYn-4 :
MYn-4
Mn+ + EDTA
After the equivalence point :
There is excess EDTA and virtually all the metal
ion is in the MYn-4 form
Example :
Calculate the shape of the titration curve for the
reaction of 50.0 ml of 0.0500 M Mg2+ (buffered to
pH10.0) with 0.0500 M EDTA (Kf = 6.2 x 108 ; at
pH10.0 : aY = 0.36)
Mg2+ + EDTA
Using
Kf’ = a4Kf
MgY2-
= (0.36)(6.2 x 108)
= 2.2 x 108
Since Kf’ is large
the reaction goes to
completion with each addition of titrant
Before equivalence point :
Consider the addition of 5.0 ml EDTA to the solution
Moles of EDTA added = (0.005 l)(0.0500 M)
= 2.5 x 10-4
Moles of Mg2+ present initially= (0.050 l)(0.050M)
= 2.5 x 10-3
Moles of Mg2+ present after the addition of EDTA =
(2.5 x 10-3 ) – (2.5 x 10-4) = 0.00225
[Mg2+] =
0.00225
0.055
= 0.0409 M
pMg2+ = -log [Mg2+] =1.39
At equivalence point : [Mn+] = [EDTA]
Volume of EDTA added = (2.5 x 10-3)/(0.0500M)
= 50.0 ml
Since there is negligible dissociation,
[MgY2-]
=
0.0025 mol
0.100 l
= 0.025 M
From:
Mg2+ + EDTA
MgY2-
Let x =[Mg2+] = [EDTA], then
Kf’ =
[MgY2-]
[Mg2+][EDTA]
=
0.025 - x
x2
= 2.2 x 108
x = 1.07 x 10-5 M
pMg2+ = -log [Mg2+] =4.97
After equivalence point :
If 51.0 ml of EDTA is added
there will be
1.0 ml excess EDTA in the solution
[EDTA] ={(0.001)(0.05M)}/(0.101 l)
=4.95 x 10-4 M
[MgY2-] =(2.5 x 10-3)/(0.101 l)
=2.48 x 10-2 M
Using
[MgY2-]
Kf’ =
[Mg2+][EDTA]
= 2.2 x 108
[Mg2+] = 2.3 x 10-7 M
p[Mg2+] = -log [Mg2+]
= 6.64
=
2.48 x 10-2
[Mg2+](4.95 x 10-4)
Metal Ion Indicators
Methods to detect the end point in EDTA titrations
are :
- metal ion indicators
A metal ion indicator is a compound whose color
changes when it binds to a metal ion (eg
Eriochrome black T)
This compound must bind metal less strongly than
EDTA
MgIn + EDTA
(red)
(colorless)
MgEDTA + In
(colorless) (blue)
- mercury electrode : measurement to potential
- glass pH electrode
- ion-selective electrode
EDTA Titration
Techniques
(i) Direct Titration
-
analyte is titrated with standard EDTA
-
analyte is buffered to an appropriate pH at
which the conditional formation constant for
the metal-EDTA complex is large and the
color of the free indicator is distinctly
different from that of the metal-indicator
complex
(ii) Back titration
-
a known excess of EDTA is added to the
analyte
-
the excess EDTA is titrated with a standard
solution of a second metal
-
this method is useful if
- analyte precipitates in the absence of
EDTA
- the analyte reacts too slowly with EDTA
under titration conditions
- analyte blocks the indicator
-
metal used in back titration must not
(iii) Displacement titration
Method is useful is the metal ions do not have a
satisfactory indicator
- analyte is treated with excess Mg(EDTA)2- to
displace Mg2+
Mn+ + MgY2-
MYn-4 + Mg2+
- the displaced Mg2+ is titrated with standard
EDTA
Example :
2Ag+ + Ni(CN)42-
2Ag(CN)2- + Ni2+
(iv) Indirect titration
Anions (such as SO42-, CrO42-, CO32- and S2-)
that precipitate with certain metal ions can be
analyzed with EDTA through indirect titration
For example, SO42 can be precipitated with
excess Ba2+. The BaSO4(s) is washed and
boiled with excess EDTA at pH10 to bring Ba2+
back into solution as Ba(EDTA)2-. The excess
EDTA is back titrated with Mg2+